Conventional fluorescent tubes used in conventional luminaire housings have a straight, crescent or circular body with a maximum length of 2400 mm of the tube. Fluorescent tubes normally are low-pressure discharge lamps having a coating on the inner surface comprising a fluorescent material such as phosphor. The fluorescent tube lamp typically comprises an air-tight glass tube, a fill of inert gas, and electrodes. At each end of the fluorescent tube, there is a lid with two symmetrically positioned contact pins, to which the electrodes are connected. Electric power supply is provided to the fluorescent tube via these two contact pins.
Replacing a fluorescent tube, e.g. for the purpose of energy saving, with a retrofitting LED lamp (afterwards LED lamp), is becoming common nowadays. Retrofitting is understood to mean replacing a fluorescent tube by a LED lamp without altering the luminaire housing of the fluorescent tube. The luminaire housing comprises a base, at least two tube holders as well as the electronic devices necessary for operating the fluorescent tube. Changing the fluorescent tube may not include the optional requirement of removing or replacing the starter or a load or ballast by something else.
When replacing a fluorescent tube with a LED lamp, an issue related to the threatening of an electric shock during the assembly process of the LED lamp can occur. According to the safety regulations in the field of electricity, luminaire housings are constructed such that when a fluorescent tube is replaced, it is not possible to touch any voltage-carrying parts even if the fluorescent tube housing is not disconnected.
This requirement has also to be adhered even if the fluorescence tube is replaced in such a way that only one end of the tube is in contact with the contacts of a tube holder of the luminaire housing so that the person replacing the tube can touch the other end of the tube. This requirement is met automatically with a fluorescent tube because no current flows through the gas-filled fluorescent tube before the gas in the tube is ionized by a voltage pulse. This starting pulse is generated by a so called ballast. In other words, unless being ionized, the gas in the fluorescent tube is nonconductive. Thus, the electric structure of the luminaire housing is such that the generation of a starting pulse is required to electrically connect both ends of the fluorescent tube to each other. Hence, the fluorescent tube prevents the risk of an electric shock during replacement by means of constructive measures.
With LED lamps, this electric safety requirement is not automatically met. LED lamps usually comprise a printed circuit board or a corresponding structure, on which LEDs and other electronic components and drivers such as driver devices for the LEDs are mounted. The purpose of the components is to convert the alternating voltage of the power supply into direct voltage and to control the directed current required by the LEDs. In practice, if one end of the LED lamp is connected to the tube holder of the luminaire housing the LED lamp is energized. In other words, the LED lamp may always be in a conductive state without having been supplied with a starting pulse by the ballast. Therefore, when the LED lamp is being mounted on fluorescent luminaire housings, the contact pins at one end of the LED lamp may be connected to the contacts of one tube holder of the fluorescent luminaire housing while the other end of the LED lamp may still remain outside the fluorescent luminaire housing. In consequence, a person mounting or replacing the LED lamp may touch the free pins of the LED lamp which are under voltage.
US 2011/0260614 discloses a LED lamp for replacing a fluorescent tube. The LED lamp comprises a safety unit to prevent a voltage from transferring through the LED lamp from its one end to the other end until a voltage supplied from a corresponding tube holder of the luminaire housing to the pair of contact pins has been separately detected at each end of the LED lamp. Inside the LED lamp, there is at least one optical line that is arranged to transfer a control or measurement signal associated with the safety unit from one end of the LED lamp to the other without capacitive leakage currents. However, the safety unit is expensive and may itself show a malfunction causing an electric shock.